Thermostatic switch



Feb. 13, 1962 R, D, GRAF ETAL 3,021,406;

THERMOSTATIC SWITCH Filed NOV. 16, 1959 FIG.3

INV EN TORS AGEN T yRobert D.

ming assembly `and adjustment of the switch means;

United States Patent e 3,021,406 f rHERMosrATic SWITCH Wire Corporation f Filed Nov. 16, 1959, Ser. No. 853,101 Claims. (Cl. 20D-138) Graf, Flora, and William M. Parkins andr Robert E. `Prouty, Logansport, Ind., assignors to Essex l be engageable with'lthe frame to'limit theA overcenter This invention relates to thermostatic switches and.`

more particularly to miniature thermostatic switches of the snap-acting type which may be hermetically sealed in a protective casing.

It is common practice to protect electric motors against overload conditions by various types of small thermostatic switches which are incorporated in the windings of the motors to be responsive to the temperature of themotor windings as well yas to the electric current ofthe windings. In order to be embedded directly in thepmotor i windings, these thermostatic switches must bequite small, usually not appreciably over two inches in length or over three-eighths (3/s) yinch in diameter. Because of this vsize requirement, such thermostatic 'switches are ordinarily of the creep type comprising merely a fiat strip of bimetal carrying'a movable contact cooperating with.

a fixed contact secured'to a tubular casing in which th switch parts are hermetically sealed. v

Although the advantages of snap-acting thermostatic switches have been long recognized,eiforts to construct] miniature switches of the foregoing 4type merely by re` ducing thesizeL of the component parts of prior designs and enclosing them in sealed housings have not been entirely satisfactory. Besides the ditliculty of enclosing such .constructionsin 1a small tubular casing, some of switch comprising a cylindrical casing 10 surrounding a( cylindrical frame 11 dimensioned to closely fit within the Vmay be provided with' openings intermediate its ends movement after separation of the contacts. k

Other Lobjects and features ofthe invention' will be- 'come apparent from reference tothe following description and accompanying drawing, in which:

switch embodying the 'present' invention;

' FIG; lis a plan view of a snap-acting thermostatic FIG. 2 is a side-elevational view of the switch prior FIG. 6 is a view similar to FIG.3, but illustrating a slightly modified embodiment of the invention,

Referring to the drawing, there is shown a thermostatic casing 10.r Casing 410 and frame 11V are 'of metal capable rof being soldered such as steel andmay be formed from seamless tubing. As an aid to assembly and adjustmentof the various component parts of the switch, frame `1.1

' such as holes r12 and slots 13.

One end of the frame is counterbored' atv14 to receive a terminal header assembly 15 having a central terminal loof conductive metalV supported within an outer metal y sleevell by a glass bushing r18 which'is bonded to both ',theterminal 1"6 and the sleeve v17.

such switch constructions are diiiicult to assemble orf accurately calibrate and are excessively costly. Therevduction-in size also often has an adverse eifecton vthe durability of such thermostatic switch constructions.

"1j Accordingly, it is an object of this invention to provide a new and improvedthermostatic switch of miniature size which can be easily hand Ieconomically manufactured,

which is durable'` in operation and which may be hermeti.- cally sealed; f v

Another object of the invention is to provide newand improved thermostatic switch of miniature size whichM operates with a snapv action. v

A further object of the invention is to provideV anew and improved thermostatic switch of miniature size which 1 may be readily and permanently calibrated. m

lIn the preferred embodiments of the invention hereinafter described, the component parts ofa thermostatic switch means are disposed within a tubular metal frame provided with apertures intermediate itsv ends facilitat- An adjusting screw `is threaded in one end of the frame and adjusted to calibratethe switch means. The other end.

of the frame is closed by a terminal header having an outer metal sleeve and an insulated terminal connected to the switch means.4 The frame is positionedlwithin a closely fitting metal tubular casing and is sealed thereto. As disclosed, the thermostatic switch means comprise a bimetallic controlling element 'and a spring contact arm which are secured together lon the header terminal. The free end of the bimetallic element isy connected tothe terminallti is attened at519pto provide a support to which -aspringl contact arm 20.and a bimetallic actuat' Y ingelementzl are rigidly ksecured as by spotwelding.

1 Spring arm 20 is formed from 'exible conductive metal..

such as a beryllium-copper'alloy and carriesy a movable electrical contact 22 .near its free end... Spring arm20 'is biased as by bending flattened end 19 of terminall16 to normallyr urge contact 22 into engagement with a fixed contact 23 vwhich is, welded to the inner surface of the frame 11.. Contacts 22 and 23 preferably comprise a facing of silver'alloy over a steel backing.

A driving element 24 of any suitable insulating matef rial is secured'to the free end of vspring arm 20 by"an eyelet 25 and has a shank portion 2,6 extending from one side of spring arm 20 through a hole 27 in bimetallic element 271. The free end of driving element 24V is provided with a shoulder or stop 2,18 for limiting movement ofbimetallicelement 21 relative to spring arm 20.` Shank portion 26 passe'sffreely through holel 27 and it will be.

noted that driving elementZft provides alost motion connection between spring arm 2l) and bimetallic element 21. Bimetallic element 21 is composed of superposed layers fof metals having different coeicients of expansion such f that when it is heated its freeend will detiect in a direcadjusting screw by an overcenter means comprising a` bearing member pivotally seated on the `bimetallic element and `a coil Vspring confined between the bearing member and the screw. The spring arm carries acon-y tact cooperable with a fixed contact carried by the frame and has a lostmotion connection with thebimetallc element providing for separation of the contacts withy a" snap action. bearing rmember is constructed to tion away from spring arm 20. The free end of bimetallic element 21 terminates'in sharp edges 29 on opposite sides of al narrow tonguek 30. Sharp edges 29 are adapted to engage abearing surface 31 defined byv an elongated V-shaped'. notch 32 formediin a bearing member 33 of a suitable insulating material such as ceramic. Bearing surface 31 has a central recess 34 to loosely receive tongue 30 and thereby prevent accidental disengagement of bearing member 33 from bimetallic element 21. Bearing 'member33 is also formed with a lug 35 for supporting one end of a coil spring 36. The other end of coil spring 36 is seated on the conical extension 37y of an adjusting screw 38 ywhich is provided with a threaded portion 39 ditted in the threaded inner surface 40 of frame 11. The

Patented 13,l 19,62z

The inner end ofk adjusting screw 38 may be'provicled with a stem 41 by which screw 38 may be turned to adjust the compression of coil spring 36. Screw 3S :is made of a soldera-ble conductive metal and stem 41 may also serve as a terminal connection to contact 23 through Vframev 11.

From the foregoing description, itswill be apparent that bearing member 33 and coil spring 36 provide an overcenter` connection between' bimetallic elementV 21 and screw 38. With thei parts in theposi'tion shown by FIG. 2, coil spring 35 urges the'free` end of bimetallic element 21 against spring arm 29 thereby cooperating with spring arm20 in `forcing contact 22 into engagement with contact 23. `In response'tovanincrease in ambient temperature and/ or to the heating elect of the passage of current through spring arm 2t) bimetallic element 21 will dei'lect in a direction-away from spring arm 2li. As the bimetallic element 21 moves away from spring arm 20, the free end of bi'rnetallic element 21 is moved toward a position of unstable equilibrium, whereupon coil spring 36 acts to continue the movement of bimetallic element 21 in the same direction :with a snap action until bearing member 33 engages'the inner surface of frame 1K1. As bimetallic element 21 moves relative to lspring arm 2G with a snap movement, it Ymoves into engagement with shoulder 2B of driving element 24 with a hammer blow'and exes spring arm -20 to quickly separate contact 22 from contact 23. Under this condition of operation, bimetallic element 21 does not engage driving element 24 until after initiation of the overcenter snap movement. After completion of the switching operation, the parts of the switch assume the position shown in FIG. 3. A

When the bimetallic element 2 1 subsequently cools, it will dellect in a direction toward the spring arm 26 against the force of coil spring 36 until it again reaches a position of unstable equilibrium. Upon continued movement of bimetallic element 21, the end of bimetallic element 21 pivots a snap action on bearing member 33 to the position shown inY FIG. '"2. In this switch operation, engagement of bimetallic element 21 by shoulder 28 of driving element 24 prevents spring arm 21 from returning conta/ct 22'into engagement with contact 23 until after snap movement vof bimetallic element 26 has commenced.

Contact 22iswt'hus causedto move into engagement with contact-23 with asnap'action. I

`From the foregoing description of the switch construction, it can beissen-thatV the switch may be simply assembled and adjusted. The assembly consisting of spring arm 20, contact 22, bimetallic element 21 and driving element 24' may be assembled lirst and then welded to the flattened end 19 of terminal `16 or' the header 15. After contact 23 iswelded to frame 11 and the foregoing assembly is Vsecuredin the frame 11 by welding the sleeve 17 of header 15in the end of frame 11, the force with which spring arm 20 urges contact 22 against contact 23 may be adjusted by means of a vsuitable instrument inserted through holes 1,2 in frame 11. The bearing member 33 with coil 'spring 36`placed on its lug 35 is next assembled in pivotal engagementwith the end of bimetallic element 21 and adjusting screw 38 is screwed into the threaded end of frame 11. It will be apparent that the assembly of the foregoing is greatly facilitated by the use of the slots 13 in frame 11. Adjustment of the kswitch is then completed by .turning adjusting screw 38 to cause snap opening of the contacts when the'bimetallic element 21 is heated to the desired temperature. After the above assembly and adjustmenthave been completed, the frame 11 is inserted in casing 1-0 and both ends of the assembled switch are clipped in molten solder to hermitically seal the switch. .At one end of the switch, the-'solder seals the adjusting screw 3S to the frame 11 and simultaneously seals the end of theframe 11 to the adjacent end of the casing 1li. At the other end of the switch, the

solder seals ythe sleeve 17 of terminal header 15 to the frame 1,1 KYand simultaneously seals the end of the frame 1110 theend .Ofithe casing .10.. y

In the modified embodiment ofthe invention illustrated in FG. 6, the exterior end of adjusting screw 38a is provided with a sot Si? instead'of a stern to permit casing lila to be closed at this end. With either this construction or that previously described, it is clear that a Wiring terminal could be secured directly to the casing for mal:- ing electrical connection to the fixed contact. To avoid counterboring an end of the frame to receive a terminal header, the sleeve 17a of the terminalheader 15a may have a radial an'ge 51 which abuts the end of frame 11a and which is abutted by the end of casing 10a to position these parts. Flange 51 may be soldered or welded to casing 10a to hermetically seal the switch. A rigid bracket 52 is attached to the header terminal 16a and carries a calibrating screw 53 for biasing spring arm 29a and birnetallic element 21a toward contact 23a. This arrangement eliminates the bending of the header terminal previously described and thus avoids possible breakage of the header bushing 18a. It will be obvious that the construction and operation of the switch of FIG. 6 are similar to those of the switch which has been already described.Y

While specific embodiments of the invention have been shown and described, itis to be understood that the invention is not to be limited to the precise details herein shown and described since the same may be carried out in other ways falling within the scope of the invention as claimed.

What is claimed is: Y

l. A miniature thermostatic switch comprising a tubular elongated frame ofY conductive metal; a fixed support mounted within said frame and insulated therefrom; a spring arm Vdisposed within said frame; a bimetallic element disposed within said frame; said spring arm and bimetallic element being secured to said support and extending therefrom inthe same direction toward the second end of said frame; a fixed contact conductively secured to said frame; a movable contact carried on said spring arm for'cooperation with said fixed contact; an adjusting screw threaded in said second end of the frame; a bearing member of insulation material pivotally seated upon the free end of rsaid bimetallicy element; ovetcenter spring meanscomprising acoil spring contined4 under compression between said screw` and said bearing member; lost motion means connecting said spring .army and said bimetallic element to impart a .snap action to said Y spring arm when said bimet'allicl element snaps over center; and said bearing member having a portion engagevablel with said frame to limit overcenter 'movement of said bimetallic element in one direction.

s 2. The thermostatic switch laccording to claim vl wherein said lost motion means comprise a driving element of insulating material secured to said spring arm; rsaid driving element having a shank portion extending from one side of said spring arm through said bimetallic element and being provided at one end with a shoulder spaced from said spring arm.

3. A miniature, hermetically sealed thermostat comprising, in combination; arcylindrical metal frame open at both ends; stationary contact means conductively secured to the inner surface of said frame; a terminal header closing one end of said framej a terminal passing through said header and electrically insulated from said frame; movable contact means electrically connected lto said terminal and cooperating with said .stationary contact means; a thermally responsivev bimetallic element having one end fixed to the inner end of said terminal; the other end of said bimetallic element being movable in response to itemperature changes for actuating said movable contact means; an. adjustable screw threaded in the other end of said frame; a bearing member of insulation pivotally seated upon said movable end of the bimetallic element; overcenter spring means comprising a coil spring confined under compression between said screw and said bearingmember for imparting snap action to movement of saidbimetallic element; said bearing member having akportion engageable with the inner surface of said frame to limit vthe movement of said bimetallic element in a contact-opening direction; said frame being provided `with an opening in the wall thereof adjacent said bear` 6 endysaid closedendof the casing beingrpositioned over said other end of the frame; said open end of the casing i adjoining said one end of the frame; said header having ing member and spring means for providing access there-K to during their assembly in said frame; a cylindrical metal casing telescopically'positioned over said frame in close fitting relation thereto; and said casing completely covering said opening in the frame and being secured to the frame by fused metal.

4. The thermostatic switchl according to claim 3 Wherein said casing is open at both ends; said screw being sealed in the frame by fused metal. Y

5. The thermostatic switch according to claim 3 Wherein said casing is closed at one end and open at the other` anfouter sleeve of metal provided withr an annular ange; and said flange being sealed to said open end of the casing by fused metal.

yReferences Cited in the file of this patent UNITED STATES 15 ATE1-JTS 1,602,876y Bradshaw oct. 12, 1926 2,402,312 Burch June r18, 1946 n FOREIGN PATENTS Y 650,997 France '-1 1 Oct. 2, 1928 566,284 Germany i.- Dec( 15, 1.932

` France Feb. 25,` 1953 

